1. Field of Invention
The present invention relates to a centrifugal switching device of a single-phase induction motor.
2. Related Art
Since single-phase induction motor has a simple but firm structure, and a single-phase power supply can be easily found, the single-phase induction motor serving as a driving motor for electrical appliances is widely applied in family, office, industry, and agriculture. However, the motor cannot activate automatically. Therefore, in order to generate a starting torque, besides an operating winding, a starting winding having a phase difference with the operating winding should be further provided. When the rotating speed of a rotator reaches 70%-80% of the synchronous speed, a centrifugal switching device is activated to disconnect the starting winding.
The centrifugal switch has an operating winding and a starting winding. The starting winding has a large resistance, and a start switch is serially-connected. When the rotating speed reaches 75%-80% of the synchronous speed, the centrifugal switch is turned off to disconnect the starting winding from the power supply. The operating winding has a small resistance and a large reactance, and the starting winding has a large resistance and a small reactance. Under different impedance ratios of the two-phase winding, the two-phase current has different phases, and thus a rotating magnetic field is generated to activate the motor with a required starting torque.
Hereinafter, the conventional centrifugal switching device of a single-phase induction motor is illustrated with the accompanying drawings.
Referring to FIGS. 1 and 2, the centrifugal switch driving device of a single-phase induction motor includes a sleeve 20, a slider 30, a centrifugal switch 60, a pair of governor weights 40, and a pair of springs 50. The sleeve 20 is rotatably integrated with a spindle 10 of the single-phase induction motor. The slider 30 is rotatably integrated with the sleeve 20 and formed with a collar 32 of an expanded diameter, and the collar 32, when sliding in an axial direction of the spindle 10 with respect to the sleeve 20, contacts a movable control rod 62 of a centrifugal switch 60, so as to turn on/off the centrifugal switch 60. The centrifugal switch 60 forms an open circuit from a secondary coil to a primary coil enwinding the stator of the single-phase induction motor due to the operation of the slider 30. The pair of governor weights 40 is joined to the slider 30 and capable of moving with respect to the same, and rotate under a centrifugal force generated by the rotation of the spindle 10, so as to force the slider 30 to move in the axial direction with respect to the sleeve 20. The pair of springs 50 has one end joined to the governor weights 40 respectively, and provides an elastic force in a direction of drawing the governor weights 40 closer.
The centrifugal switch 60 includes a housing 61 with a certain internal space, a movable control rod 62, a spring 63, a first leaf spring switch control rod 65, a second leaf spring switch control rod 67, a secondary coil terminal 68, a heater power terminal 69a, and a heater power terminal 69b. The movable control rod 62 is constituted by a hinge axis 62a hinged to the housing 61 and serving as a reference, a collar contact portion 62b formed on an opening portion of the housing 61 and exposed for contacting the above collar 32, and a contact driving portion 62c protruding toward the internal side of the housing 61. The spring 63 is used for elastically supporting the contact driving portion 62c of the movable control rod 62. The first leaf spring switch control rod 65, mounted adjacent to the contact driving portion 62c, has one side fixed to a primary coil terminal 64, and protrudes out a certain length in order to gain elasticity. The second leaf spring switch control rod 67, mounted adjacent to the contact driving portion 62c, has one side fixed to a heater power terminal 66, and protrudes out a certain length to gain elasticity. The secondary coil terminal 68 and the heater power terminal 69a are correspondingly mounted at two sides of the other distal end of the first leaf spring switch control rod 65, and are controlled by the first leaf spring switch control rod 65 to turn on/off. The heater power terminal 69b is mounted adjacent to the other distal end of the second leaf spring switch control rod 67, and is controlled by the second leaf spring switch control rod 67 to turn on/off.
Moreover, in order to turn on/off the first leaf spring switch control rod 65 and the second leaf spring switch control rod 67, a pressure should be exerted. Therefore, at an distal end of the above contact driving portion 62c, a pressing protrusion 62d protruding a certain height is respectively formed at a position adjacent to the first leaf spring switch control rod 65 and the second leaf spring switch control rod 67.
Based on the above structure, the conventional centrifugal switch driving device of a single-phase induction motor operates in the following manner.
When the spindle 10 stops, under the elastic force of the spring 50, the governor weights 40 are set at an original position close to the spindle 10, and the slider 30 is disposed at a position far from a guide device. At this time, as shown in FIG. 2, the collar contact portion 62b of the movable control rod 62 remains in a pressed state via the collar 62. Moreover, as the pressing protrusions 62d formed at the distal end of the contact driving portion 62c respectively exert a pressure on the first leaf spring switch control rod 65 and the second leaf spring switch control rod 67, the distal ends of the two control rods are respectively connected to the secondary coil terminal 68, such that the primary coil terminal 64 is connected to the secondary coil terminal 68. Thereby, a power is supplied to the primary coil and the secondary coil, and the spindle 10 starts rotating.
Once the rotating speed of the spindle 10 reaches 70%-80% of the synchronous speed, the centrifugal force exerted on the governor weights 40 is larger than the pulling force of the spring 50. Thus, one side of each governor weight 40 moves outward along with the guide device, and meanwhile the other side movable with respect to the slider 30 moves toward the guide device. In this manner, the slider 30 moves in the axial direction of the spindle 10 toward the guide device.
At this time, the collar 32 originally exerting a pressure on the collar contact portion 62b of the movable control rod 62 is removed, so as to release the collar contact portion 62b from the pressed state. Moreover, under the elastic force of the spring 63 for elastically supporting the contact driving portion 62c, the movable control rod 62, as shown in FIG. 2, rotates counterclockwise.
Along with the rotation of the movable control rod 62, the pressing protrusions 62d formed at the distal end of the contact driving portion 62c also release the pressure exerted on the first leaf spring switch control rod 65 and the second leaf spring switch control rod 67 respectively. Therefore, the first leaf spring switch control rod 65 and the second leaf spring switch control rod 67 moves toward the movable control rod 62 under the elastic forces of their own. At this time, the distal end at the other side of the first leaf spring switch control rod 65 assumes a turn-off state relative to the secondary coil terminal 68 and assumes a turn-on state relative to the heater connecting terminal 69a, and the second leaf spring switch control rod 67 turns from a turn-off state to a turned-on state relative to the heater connecting terminal 69b. That is, when turning into an open-circuit state, the primary coil and the secondary coil assume a closed-circuit state relative to the heater connecting terminal 69a, and thus the spindle 10 continues to rotate only by the power provided for the primary coil.
In view of the above, in the conventional centrifugal switch of a single-phase induction motor, when the motor rotates and reaches a certain rotating speed, the governor weights used for balancing the weight stretch in the radial direction under the centrifugal force, and thus the movable collar slides along the spindle. Due to the movement of the movable collar, the movable control rod of the centrifugal switch is driven to operate the switches in the housing, and as the governor weights stretching in the radial direction under the centrifugal force are pulled by the springs during the operation, once the motor stops, the governor weights will restore to their original positions under the elastic restoring force of the springs. In the above processes, the following problems may occur. As the movable control rod 62 always operates at one side of the circle, the slider 30 may be easily seized during the actuation, and in the long run, the spring may rupture or the governor weights may generate noises. Further, the structure of the movable control rod 62 is complicated and constituted by a plurality of different parts, so during the assembling of the whole movable portion, additional processes are needed, thus prolonging the assembling time and reducing the productivity.
Meanwhile, the conventional product also has a problem that, electric sparks may easily occur due to a long switching time during the operation of the first leaf spring switch control rod and the second leaf spring switch control rod in the switching structure, and a large transient current will be generated to damage the motor and even cause fire.